Drop-in resolution
Seiko NH34 (TMI NH34A) vs Seiko 4R34: do they swap?
Whether the Seiko NH34 (TMI NH34A) and Seiko 4R34 share a case, and what changes if you move between them.
Interchangeable. The Seiko NH34 (TMI NH34A) and Seiko 4R34 share the same 27.40 mm x 5.32 mm footprint and the same centre seconds, GMT 24h hand, date at 3 layout, so one drops into the other's case with the correct dial and hands.
The Seiko NH34 (TMI NH34A) is the affordable modding GMT automatic, adding a caller-GMT hand to the NH35 base. The Seiko 4R34 is the branded Seiko caller-GMT automatic, the retail version of the NH34.
Their published figures are the same to the tenth of a millimetre. The GMT member of the NH3x family at 27.40 mm x 5.32 mm, adding a 24-hour hand to the NH35's date layout. It shares the NH35 case footprint and stem, so it drops into an NH35 case but needs a GMT dial and hand set. It is the unbranded Seiko 4R34.
Matching diameter and height is necessary but not sufficient. A donor is only a true drop-in when the dial-foot positions, hand-hole sizes, stem length and calendar layout also match. Two movements can share a case and still need a new dial, re-broached hands or a cut-to-length stem.